Non-invasive imaging technologies allow images of the internal structures of a patient or object to be obtained without performing an invasive procedure on the patient or object. In particular, technologies such as computed tomography (CT) use various physical principals, such as the differential transmission of x-rays through the target volume, to acquire image data and to construct tomographic images (e.g., three-dimensional representations of the interior of the human body or of other imaged structures).
Modern CT systems include automatic exposure control (AEC) configured to set the dose operating point to achieve optimized performance between image quality and radiation dose. Radiation exposure from CT scanners is measured in terms of the volumetric CT Dose Index (CTDIvol). Although governed by the size of the patient anatomy, the CTDIvol metric does not consider patient size, and so CTDIvol is related to x-ray tube output, as determined by the employed scanning technique, rather than individual patient size.
In an effort to create a practicable and patient-centered dose metric, a Task Group from the American Association of Physicists in Medicine (AAPM TG #204) proposed using an improved patient size-specific dose metric that modifies the CTDIvol of scan with reference to estimated patient size; this size-specific dose estimate (SSDE) metric (a metric related to the patient's attenuation and size) has been found to be an improved surrogate for actual patient absorbed dose, when compared to CTDIvol. Current embodiments of the SSDE metric are determined after scan data is acquired by calculating dimensions from tomographic axial images, i.e. the effective patient diameter Deff. Since the Deff is currently derived from reconstructed axial images of the patient, the SSDE is a post hoc dose metric and cannot govern the a priori patient exposure for a clinical imaging task.